linux-kernel - [PATCH v4 32/41] arm_mpam: resctrl: Update the rmid reallocation limit

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Message-ID: <20260203214342.584712-33-ben.horgan@arm.com>
Date: Tue,  3 Feb 2026 21:43:33 +0000
From: Ben Horgan <ben.horgan@....com>
To: ben.horgan@....com
Cc: amitsinght@...vell.com,
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	Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: [PATCH v4 32/41] arm_mpam: resctrl: Update the rmid reallocation limit

From: James Morse <james.morse@....com>

resctrl's limbo code needs to be told when the data left in a cache is
small enough for the partid+pmg value to be re-allocated.

x86 uses the cache size divided by the number of rmid users the cache may
have. Do the same, but for the smallest cache, and with the number of
partid-and-pmg users.

Tested-by: Gavin Shan <gshan@...hat.com>
Tested-by: Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Tested-by: Peter Newman <peternewman@...gle.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>
Signed-off-by: James Morse <james.morse@....com>
Signed-off-by: Ben Horgan <ben.horgan@....com>
---
Changes since v2:
Move waiting for cache info into it's own patch

Changes since v3:
Move check class is csu higher (just kept to document intent)
continue -> break

to squash update rmid limits
use raw_smp_processor_id()
---
 drivers/resctrl/mpam_resctrl.c | 39 ++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
index e8102cc74de8..e2b1afca5f01 100644
--- a/drivers/resctrl/mpam_resctrl.c
+++ b/drivers/resctrl/mpam_resctrl.c
@@ -486,6 +486,42 @@ void resctrl_arch_reset_rmid(struct rdt_resource *r, struct rdt_l3_mon_domain *d
 	reset_mon_cdp_safe(mon, mon_comp, USE_PRE_ALLOCATED, closid, rmid);
 }
 
+/*
+ * The rmid realloc threshold should be for the smallest cache exposed to
+ * resctrl.
+ */
+static int update_rmid_limits(struct mpam_class *class)
+{
+	u32 num_unique_pmg = resctrl_arch_system_num_rmid_idx();
+	struct mpam_props *cprops = &class->props;
+	struct cacheinfo *ci;
+
+	lockdep_assert_cpus_held();
+
+	if (!mpam_has_feature(mpam_feat_msmon_csu, cprops))
+		return 0;
+
+	/*
+	 * Assume cache levels are the same size for all CPUs...
+	 * The check just requires any online CPU and it can't go offline as we
+	 * hold the cpu lock.
+	 */
+	ci = get_cpu_cacheinfo_level(raw_smp_processor_id(), class->level);
+	if (!ci || ci->size == 0) {
+		pr_debug("Could not read cache size for class %u\n",
+			 class->level);
+		return -EINVAL;
+	}
+
+	if (!resctrl_rmid_realloc_limit ||
+	    ci->size < resctrl_rmid_realloc_limit) {
+		resctrl_rmid_realloc_limit = ci->size;
+		resctrl_rmid_realloc_threshold = ci->size / num_unique_pmg;
+	}
+
+	return 0;
+}
+
 static bool cache_has_usable_cpor(struct mpam_class *class)
 {
 	struct mpam_props *cprops = &class->props;
@@ -991,6 +1027,9 @@ static void mpam_resctrl_pick_counters(void)
 			/* CSU counters only make sense on a cache. */
 			switch (class->type) {
 			case MPAM_CLASS_CACHE:
+				if (update_rmid_limits(class))
+					break;
+
 				counter_update_class(QOS_L3_OCCUP_EVENT_ID, class);
 				break;
 			default:
-- 
2.43.0